The present invention is directed to a polishing pad for the chemical-mechanical planarization of semiconductor wafers and a method of making it. Semiconductor wafers may have multiple layers of wiring devices on a single wafer. These wiring devices consist of hundreds of electrical circuits fabricated and interconnected in order to produce the computer chips that will eventually be die cut from the wafer. These wiring devices are called integrated circuits (IC). A layer of insulating materials, often silicon dioxide (S1O2), separate each layer of integrated circuits so that designated IC's interconnect. In order to pack more devices into less space, the requirements for feature size within the IC's has shrunk dramatically. There may now be feature sizes smaller than 0.01 microns. As layers of integrated circuits and insulating layers are deposited, one on the other, it is of utmost importance to maintain the wafer surface on each layer in an extremely flat condition. Features that protrude into another layer and make contact where not intended or do not make contact where intended can cause short circuits, open circuits and other defects that make a valuable product unusable.
The most effective method of polishing and planarizing a multilayer integrated circuit devices is chemical-mechanical planarization (often times called polishing), or CMP. When a layer of metal interconnects or insulation is put down, it must be polished flat; that is, it is planarized before the next layer is deposited. Otherwise, small surface irregularities can cause defects, and an extremely valuable part can be defective and lost. As each layer is deposited and planarized, multiple layers are successfully built up as needed for a particular device.
Chemical-mechanical planarization is superior to previously used technologies because it has proven capable of both local and global planarization of the materials used to build multi-level integrated circuit devices. In this process, a slurry of fine abrasive particles in conjunction with chemicals that attack the surface being polished are used together with a mechanical polishing process to achieve the necessary degree of flatness prior to the deposition of the next layer.
One problem with this approach has been changes in the rate of removal over the life of the polishing pad. Most conventional polishing pads in use at present consist of polyurethane-cast resin; polyurethane fibers are impregnated with polyurethane or a combination thereof. The polishing surface of these pads tends to become glazed and worn over time during the polishing operation on multiple wafers. This changes the pad's surface characteristics sufficiently to cause the polishing performance to deteriorate significantly over time. This has been overcome by conditioning the pad surface during use, or between wafers as needed. This conditioning procedure removes the glazed worn surface from the pad.
The major reason that conventional polyurethane and other thermoplastic-based polishing pads require pad conditioning is that the surface of these pads undergoes plastic deformation during use. This is commonly called creep, and it is a common occurrence when thermoplastic materials are subjected to heat and pressure, however slight. This has been overcome in the semiconductor industry by pad conditioning. Pad conditioning renews the pad surface during polishing operations as required to restore original pad performance whenever this performance falls below acceptable levels. Some operations require continuous pad conditioning, others intermittent, some between wafers. Most semiconductor wafer polishing equipment includes a pad conditioning apparatus built into the equipment. This pad conditioning apparatus generally consists of an arm to which is attached a small rotating disc to which is attached the conditioning pad. This conditioning pad generally consists of fine diamond grit bonded to the bottom surface of a suitable pad material that is attached to the four-inch rotating disc. When needed, the conditioning pad traverses the polishing pad, renewing the polishing pad surface and restoring polishing pad performance. Unfortunately, pad conditioning actually removes material from the polishing pad surface so that over time the polishing pad is slowly ground away, thus shortening the polishing pad's life.
An example of a modern CMP equipment incorporating a conditioning device is shown in FIG. 1. The CMP apparatus 10 of FIG. 1 contains a lower rotating platen to which polishing pad 14 is adhesively attached. An upper rotating member is the wafer carrier 12 where the wafer 13 is retained. The slurry is introduced onto the polishing pad 14 at a point near the center of the pad. A conditioning arm 17 and attached diamond grit conditioning pad 16 traverse the polishing pad 14 in operation for renewing its surface. While this has solved the problem of glazing and subsequent variation in rate of removal, it has introduced a new problem: To wit, the shortening of pad life. Pad conditioning has significantly reduced pad life with the resulting increase in cost of ownership.
The main reasons that polyurethane pads glaze is due to their thermoplastic nature. These materials creep, (deform plastically) under heat and pressure. Even though the heat and pressures involved in CMP are not high, the thermoplastic materials will flow at the surface over time. In addition, abrasive particles from the slurry and polishing debris embed themselves into the soft surface of the thermoplastic polishing pad, creating a glazed surface that no longer retains its original polishing characteristics. Pad conditioning overcomes this at the expense of pad life.
Another problem inherent with pad conditioning systems is the cost of maintenance, and the cost of the diamond grit conditioning pads. In addition, diamond particles sometimes come loose from the conditioning pad and cause scratches on the wafer that cannot be repaired, adding substantially to the cost of ownership. Since pad conditioning reduces pad life and increases time lost for frequent pad-replacement, it is obvious that eliminating the need for pad conditioning with the attendant reduction in cost of ownership is a very desirable goal.